Reelin, very-low-density lipoprotein receptor, and apolipoprotein E receptor 2 control somatic NMDA receptor composition during hippocampal maturation in vitro

Reelin Deficiency and Synaptic Impairment in the Adolescent Prefrontal Cortex Following Initial Synthetic Cannabinoid Exposure

Background

Adolescent cannabinoid exposure can have long-lasting effects on the brain, particularly in the prefrontal cortex, where the reelin protein plays a crucial role in neural organization. Chronic cannabinoid exposure leads to reelin deficiency and behavioral abnormalities, but the underlying mechanisms remain unclear. With the increasing use of synthetic cannabinoids (SCs) among young people, understanding these effects is crucial.

Methods

We examined the cellular and synaptic consequences of initial SC exposure in adolescent male mice 1 day after a single in vivo exposure to WIN 55,212-2. Our approach combined immunohistochemistry, Western blots, conditional CB1 receptor (CB1R) knockout mouse lines, quantitative polymerase chain reaction, and ex vivo electrophysiology to investigate the effects of SC on reelin expression and synaptic plasticity. Additionally, single-molecule fluorescent in situ hybridization profiling was used to identify cellular coexpression patterns of reelin and CB1Rs.

Results

Our findings indicate that a single exposure to SC decreased reelin expression in specific prefrontal cortex layers accompanied by disrupted proteolytic fragmentation but not changes in messenger RNA expression. Single-molecule fluorescent in situ hybridization profiling revealed a strong coexpression of CB1R and reelin. Furthermore, our pharmacological and genetic approaches demonstrated that CB1Rs in GABAergic (gamma-aminobutyric acidergic) neurons mediate the SC-induced decrease in reelin. This decrease in reelin results in a reduction in long-term potentiation, phenocopying reelin haploinsufficient mice. Notably, we restored long-term potentiation by infusing reelin bilaterally, establishing a functional link between reelin depletion and synaptic deficits.

Conclusions

These findings provide new insights into the neural consequences of adolescent cannabinoid consumption and highlight the critical role of reelin in the cellular mechanisms associated with SC initiation during adolescence.

Regulatory mechanism of Reelin activity: a platform for exploiting Reelin as a therapeutic agent

Reelin is a secreted glycoprotein that was initially investigated in the field of neuronal development. However, in recent decades, its role in the adult brain has become increasingly important, and it is now clear that diminished Reelin function is involved in the pathogenesis and progression of neuropsychiatric and neurodegenerative disorders, including schizophrenia and Alzheimer's disease (AD). Reelin activity is regulated at multiple steps, including synthesis, posttranslational modification, secretion, oligomerization, proteolytic processing, and interactions with extracellular molecules. Moreover, the differential use of two canonical receptors and the presence of non-canonical receptors and co-receptors add to the functional diversity of Reelin. In this review, I summarize recent findings on the molecular mechanisms of Reelin activity. I also discuss possible strategies to enhance Reelin's function. A complete understanding of Reelin function and its regulatory mechanisms in the adult central nervous system could help ameliorate neuropsychiatric and neurodegenerative disorders.

Elevated insulin growth factor-1 in dentate gyrus induces cognitive deficits in pre-term newborns

Prematurely born infants are deprived of maternal hormones and cared for in the stressful environment of Neonatal Intensive Care Units (NICUs). They suffer from long-lasting deficits in learning and memory. Here, we show that prematurity and associated neonatal stress disrupt dentate gyrus (DG) development and induce long-term cognitive deficits and that these effects are mediated by insulin growth factor-1 (IGF1). Nonmaternal care of premature rabbits increased the number of granule cells and interneurons and reduced neurogenesis, suggesting accelerated premature maturation of DG. However, the density of glutamatergic synapses, mature dendritic spines, and synaptic transmission were reduced in preterm kits compared with full-term controls, indicating that premature synaptic maturation was abnormal. These findings were consistent with cognitive deficits observed in premature rabbits and appeared to be driven by transcriptomic changes in the granule cells. Preterm kits displayed reduced weight, elevated serum cortisol and growth hormone, and higher IGF1 expression in the liver and DG relative to full-term controls. Importantly, blocking IGF-1 receptor in premature kits restored cognitive deficits, increased the density of glutamatergic puncta, and rescued NR2B and PSD95 levels in the DG. Hence, IGF1 inhibition alleviates prematurity-induced cognitive dysfunction and synaptic changes in the DG through modulation of NR2B and PSD95. The study identifies a novel strategy to potentially rescue DG maldevelopment and cognitive dysfunction in premature infants under stress in NICUs.

Reelin and APP Cooperatively Modulate Dendritic Spine Formation In Vitro and In Vivo

Amyloid precursor protein (APP) plays an important role in the pathogenesis of Alzheimer's disease (AD), but the normal function of APP at synapses is poorly understood. We and others have found that APP interacts with Reelin and that each protein is individually important for dendritic spine formation, which is associated with learning and memory, in vitro. However, whether Reelin acts through APP to modulate dendritic spine formation or synaptic function remains unknown. In the present study, we found that Reelin treatment significantly increased dendritic spine density and PSD-95 puncta number in primary hippocampal neurons. An examination of the molecular mechanisms by which Reelin regulates dendritic spinogenesis revealed that Reelin enhanced hippocampal dendritic spine formation in a Ras/ERK/CREB signaling-dependent manner. Interestingly, Reelin did not increase dendritic spine number in primary hippocampal neurons when APP expression was reduced or in vivo in APP knockout (KO) mice. Taken together, our data are the first to demonstrate that Reelin acts cooperatively with APP to modulate dendritic spine formation and suggest that normal APP function is critical for Reelin-mediated dendritic spinogenesis at synapses.

Extracellular matrix and synapse formation

The extracellular matrix (ECM) is a complex molecular network distributed throughout the extracellular space of different tissues as well as the neuronal system. Previous studies have identified various ECM components that play important roles in neuronal maturation and signal transduction. ECM components are reported to be involved in neurogenesis, neuronal migration, and axonal growth by interacting or binding to specific receptors. In addition, the ECM is found to regulate synapse formation, the stability of the synaptic structure, and synaptic plasticity. Here, we mainly reviewed the effects of various ECM components on synapse formation and briefly described the related diseases caused by the abnormality of several ECM components.

Adult-specific Reelin expression alters striatal neuronal organization: implications for neuropsychiatric disorders

In addition to neuronal migration, brain development, and adult plasticity, the extracellular matrix protein Reelin has been extensively implicated in human psychiatric disorders such as schizophrenia, bipolar disorder, and autism spectrum disorder. Moreover, heterozygous reeler mice exhibit features reminiscent of these disorders, while overexpression of Reelin protects against its manifestation. However, how Reelin influences the structure and circuits of the striatal complex, a key region for the above-mentioned disorders, is far from being understood, especially when altered Reelin expression levels are found at adult stages. In the present study, we took advantage of complementary conditional gain- and loss-of-function mouse models to investigate how Reelin levels may modify adult brain striatal structure and neuronal composition. Using immunohistochemical techniques, we determined that Reelin does not seem to influence the striatal patch and matrix organization (studied by μ-opioid receptor immunohistochemistry) nor the density of medium spiny neurons (MSNs, studied with DARPP-32). We show that overexpression of Reelin leads to increased numbers of striatal parvalbumin- and cholinergic-interneurons, and to a slight increase in tyrosine hydroxylase-positive projections. We conclude that increased Reelin levels might modulate the numbers of striatal interneurons and the density of the nigrostriatal dopaminergic projections, suggesting that these changes may be involved in the protection of Reelin against neuropsychiatric disorders.

The role of reelin in the pathological mechanism of depression from clinical to rodents

Major depressive disorder (MDD) is a devastating mental illness and the leading cause of disability worldwide. Previous studies have suggested that synaptic plasticity in the hippocampus plays an important role in depression pathogenesis. Reelin is expressed mainly in the frontal lobe and hippocampus, and is closely associated with neurodevelopment and synaptic plasticity. However, few studies have investigated its role in MDD combining clinical trials and animal experiments. We show that in a clinical trial, plasma reelin levels decreased in patients with first-episode drug-naïve MDD and increased after treatment; further, plasma reelin levels allowed to distinguish drug-naïve patients with first-episode MDD from healthy individuals. In rats, chronic mild and unpredictable stress led to a decrease in both reelin mRNA and protein levels in the hippocampus, which could be reversed by vortioxetine. Subsequent experiments confirmed that the reelin-ApoER2-NR2A /NR2B pathway regulates hippocampal synaptic plasticity and may be involved in depression or antidepressant responses. Our work contributes to a deeper understanding of MDD pathogenesis and provides new evidence that reelin should be considered a potential therapeutic target for MDD.

Synaptic and Genetic Bases of Impaired Motor Learning Associated with Modified Experience-Dependent Cortical Plasticity in Heterozygous Reeler Mutants

Patients with neurodevelopmental disorders show impaired motor skill learning. It is unclear how the effect of genetic variation on synaptic function and transcriptome profile may underlie experience-dependent cortical plasticity, which supports the development of fine motor skills. RELN (reelin) is one of the genes implicated in neurodevelopmental psychiatric vulnerability. Heterozygous reeler mutant (HRM) mice displayed impairments in reach-to-grasp learning, accompanied by less extensive cortical map reorganization compared with wild-type mice, examined after 10 days of training by intracortical microstimulation. Assessed by patch-clamp recordings after 3 days of training, the training induced synaptic potentiation and increased glutamatergic-transmission of cortical layer III pyramidal neurons in wild-type mice. In contrast, the basal excitatory and inhibitory synaptic functions were depressed, affected both by presynaptic and postsynaptic impairments in HRM mice; and thus, no further training-induced synaptic plasticity occurred. HRM exhibited downregulations of cortical synaptophysin, immediate-early gene expressions, and gene enrichment, in response to 3 days of training compared with trained wild-type mice, shown using quantitative reverse transcription polymerase chain reaction, immunohistochemisty, and RNA-sequencing. We demonstrated that motor learning impairments associated with modified experience-dependent cortical plasticity are at least partially attributed by the basal synaptic alternation as well as the aberrant early experience-induced gene enrichment in HRM.

Analysis of Reelin signaling and neurodevelopmental trajectory in primary cultured cortical neurons with RELN deletion identified in schizophrenia

Reelin, an extracellular matrix protein, is secreted by Cajal-Retzius cells and plays crucial roles in the development of brain structures and neuronal functions. Reductions in Reelin cause the brain dysfunctions associated with mental disorders, such as schizophrenia. A recent genome-wide copy number variation analysis of Japanese schizophrenia patients identified a novel deletion in RELN encoding Reelin. To clarify the pathophysiological role of the RELN deletion, we developed transgenic mice carrying the RELN deletion (Reln-del) and found abnormalities in their brain structures and social behavior. In the present study, we performed an in vitro analysis of Reelin expression, intracellular Reelin signaling, and the morphology of primary cultured cortical neurons from wild-type (WT) and Reln-del mice. Reelin protein levels were lower in Reln-del neurons than in WT neurons. Dab1 expression levels were significantly higher in Reln-del neurons than in WT neurons, suggesting that Reelin signaling was decreased in Reln-del neurons. Reelin was mainly expressed in γ-aminobutyric acid (GABA)-ergic inhibitory neurons, but not in parvalbumin (PV)-positive neurons. A small proportion of Ca²⁺/calmodulin-dependent protein kinase II α subunit (CaMKIIα)-positive excitatory neurons also expressed Reelin. In comparisons with WT neurons, significant decreases were observed in neurite lengths and branch points as well as in the number of postsynaptic density protein 95 (PSD95) immunoreactive puncta in Reln-del neurons. A disintegrin and metalloproteinase with thrombospondin motifs-3 (ADAMTS-3) is a protease that inactivates Reelin by cleavage at the N-t site. The knockdown of ADAMTS-3 by short hairpin RNAs suppressed Reelin cleavage in conditioned medium and reduced Dab1 expression, indicating that Reelin signaling was enhanced in the primary cultured cortical neurons of WT and heterozygous Reln-del. Accordingly, the inhibition of ADAMTS-3 may be a potential candidate in the clinical treatment of schizophrenia by enhancing Reelin signaling in the brain.

Genetic Variants and Haplotypes of Tryptophan Hydroxylase 2 and Reelin Genes May Be Linked with Attention Deficit Hyperactivity Disorder in Egyptian Children

Attention-deficit hyperactivity disorder (ADHD) has been proposed to stem from multiple etiologies, perhaps genetic in nature with biological and psychosocial motivates. Tryptophan hydroxylase 2 (TPH2) and Reelin (RELN) genes may play a key role in triggering ADHD. The purpose of this case-controlled study was to explore the linkage of the genetic variants of TPH2 and RELN genes with ADHD. One hundred Egyptian children with ADHD and 105 age and sex matched controls constituted the study samples. Genotyping was performed for TPH2 (rs11179027; rs1843809) and RELN (rs736707; rs362691) gene polymorphisms using real time PCR assay. The alleles and genotype frequencies of TPH2 and RELN gene polymorphisms were assessed in all study participants. The frequencies of the alleles of TPH2 rs11179027 (OR = 1.75, 95% CI = 1.08-2.85, p = 0.022), TPH2 rs1843809 (OR = 3.67, 95% CI = 1.82-7.43, p = <0.001), and RELN rs736707 (OR = 1.61, 95% CI = 1.03-2.51, p = 0.035) were significantly associated with ADHD, while there was no significant difference between ADHD patients and controls regarding the frequency of RELN rs362691 (OR = 1.34, 95% CI = 0.73-2.48, p = 0.34). The frequencies of CTAG, CTGG, CTAC, CTGC, and GTAC haplotypes were significantly higher in ADHD patients than in controls (p = 0.011, 0.005, 0.015, 0.001, and 0.027, respectively). In conclusion, TPH2 rs11179027, TPH2 rs1843809, and RELN rs736707 gene alleles and haplotypes might be significantly correlated with the genetic susceptibility to ADHD in Egyptian children.

Reduced Reelin Expression in the Hippocampus after Traumatic Brain Injury

Traumatic brain injury (TBI) is a relatively common occurrence following accidents or violence, and often results in long-term cognitive or motor disability. Despite the high health cost associated with this type of injury, presently there are no effective treatments for many neurological symptoms resulting from TBI. This is due in part to our limited understanding of the mechanisms underlying brain dysfunction after injury. In this study, we used the mouse controlled cortical impact (CCI) model to investigate the effects of TBI, and focused on Reelin, an extracellular protein that critically regulates brain development and modulates synaptic activity in the adult brain. We found that Reelin expression decreases in forebrain regions after TBI, and that the number of Reelin-expressing cells decrease specifically in the hippocampus, an area of the brain that plays an important role in learning and memory. We also conducted in vitro experiments using mouse neuronal cultures and discovered that Reelin protects hippocampal neuronal cells from glutamate-induced neurotoxicity, a well-known secondary effect of TBI. Together our findings suggest that the loss of Reelin expression may contribute to neuronal death in the hippocampus after TBI, and raise the possibility that increasing Reelin levels or signaling activity may promote functional recovery.

Reelin Functions, Mechanisms of Action and Signaling Pathways During Brain Development and Maturation

During embryonic development and adulthood, Reelin exerts several important functions in the brain including the regulation of neuronal migration, dendritic growth and branching, dendritic spine formation, synaptogenesis and synaptic plasticity. As a consequence, the Reelin signaling pathway has been associated with several human brain disorders such as lissencephaly, autism, schizophrenia, bipolar disorder, depression, mental retardation, Alzheimer’s disease and epilepsy. Several elements of the signaling pathway are known. Core components, such as the Reelin receptors very low-density lipoprotein receptor (VLDLR) and Apolipoprotein E receptor 2 (ApoER2), Src family kinases Src and Fyn, and the intracellular adaptor Disabled-1 (Dab1), are common to most but not all Reelin functions. Other downstream effectors are, on the other hand, more specific to defined tasks. Reelin is a large extracellular protein, and some aspects of the signal are regulated by its processing into smaller fragments. Rather than being inhibitory, the processing at two major sites seems to be fulfilling important physiological functions. In this review, I describe the various cellular events regulated by Reelin and attempt to explain the current knowledge on the mechanisms of action. After discussing the shared and distinct elements of the Reelin signaling pathway involved in neuronal migration, dendritic growth, spine development and synaptic plasticity, I briefly outline the data revealing the importance of Reelin in human brain disorders.

Quantitative Three-Dimensional Reconstructions of Excitatory Synaptic Boutons in the "Barrel Field" of the Adult "Reeler" Mouse Somatosensory Neocortex: A Comparative Fine-Scale Electron Microscopic Analysis with the Wild Type Mouse

Synapses are key structural determinants for information processing and computations in the normal and pathologically altered brain. Here, the quantitative morphology of excitatory synaptic boutons in the "reeler" mutant, a model system for various neurological disorders, was investigated and compared with wild-type (WT) mice using high-resolution, fine-scale electron microscopy (EM) and quantitative three-dimensional (3D) models of synaptic boutons. Beside their overall geometry, the shape and size of presynaptic active zones (PreAZs) and postsynaptic densities (PSDs) forming the active zones and the three pools of synaptic vesicles (SVs), namely the readily releasable pool (RRP), the recycling pool (RP), and the resting pool, were quantified. Although the reeler mouse neocortex is severely disturbed, no significant differences were found in most of the structural parameters investigated: the size of boutons (~3 μm2), size of the PreAZs and PSDs (~0.17 μm2), total number of SVs, and SVs within a perimeter (p) of 10 nm and p20 nm RRP; the p60 nm, p100 nm, and p60-p200 nm RP; and the resting pool, except the synaptic cleft width. Taken together, the synaptic organization and structural composition of synaptic boutons in the reeler neocortex remain comparably "normal" and may thus contribute to a "correct" wiring of neurons within the reeler cortical network.

Modulation of Hippocampal Gamma Oscillations by Dopamine in Heterozygous Reeler Mice in vitro

The reelin haploinsufficient heterozygous reeler mice (HRM), an animal model of schizophrenia, have altered mesolimbic dopaminergic pathways and share similar neurochemical and behavioral properties with patients with schizophrenia. Dysfunctional neural circuitry with impaired gamma (γ) oscillation (30–80 Hz) has been implicated in abnormal cognition in patients with schizophrenia. However, the function of neural circuitry in terms of γ oscillation and its modulation by dopamine (DA) has not been reported in HRM. In this study, first, we recorded γ oscillations in CA3 from wild-type mice (WTM) and HRM hippocampal slices, and we studied the effects of DA on γ oscillations. We found that there was no difference in γ power between WTM and HRM and that DA increased γ power of WTM but not HRM, suggesting that DA modulations of network oscillations in HRM are impaired. Second, we found that N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 itself increased γ power and occluded DA-mediated enhancement of γ power in WTM but partially restored DA modulation of γ oscillations in HRM. Third, inhibition of phosphatidylinositol 3-kinase (PI3K), a downstream molecule of NMDAR, increased γ power and blocked the effects of DA on γ oscillation in WTM and had no significant effect on γ power but largely restored DA modulation of γ oscillations in HRM. Our results reveal that impaired DA function in HRM is associated with dysregulated NMDAR–PI3K signaling, a mechanism that may be relevant in the pathology of schizophrenia.

Intra-day variations of blood reelin levels in healthy individuals

INTRODUCTION

Reelin (RELN) is an extracellular glycoprotein best known to be crucial for neuronal migration during the embryonic period and regulation of synaptic plasticity in the adult brain, with recent studies pointing to reelin playing an important part in the organization of peripheral organs as well. Abnormalities in RELN function are associated with a variety of medical conditions in human beings. These alterations partly also reflect in RELN's blood levels, which gives it a clinical relevance as a potential biomarker. Requirement for a possible clinical use is a profound understanding of RELN's physiology. We hypothesized blood RELN levels could underlie time-dependent variations and therefore examined individuals' serum reelin concentrations in the course of one day.

MATERIAL AND METHODS

We obtained blood samples from healthy individuals (n = 10) at several times of measurement over a time period of 24 h. We subsequently determined the respective serum RELN concentrations utilizing an enzyme-linked immunosorbent assay and tested for intra- and interindividual variations in serum RELN concentrations over time.

RESULTS

All tested individuals' serum RELN levels displayed significant intraindividual variations in the course of 24 h. Test subjects' reelin day profiles showed substantial divergence among each other.

CONCLUSIONS

Our findings point to short-term fluctuations in blood RELN levels being part of physiological RELN homeostasis. This is of special interest with regard to a potential clinical use of RELN as a biomarker.

Melanopsin Retinal Ganglion Cells Regulate Cone Photoreceptor Lamination in the Mouse Retina

Newborn neurons follow molecular cues to reach their final destination, but whether early life experience influences lamination remains largely unexplored. As light is among the first stimuli to reach the developing nervous system via intrinsically photosensitive retinal ganglion cells (ipRGCs), we asked whether ipRGCs could affect lamination in the developing mouse retina. We show here that ablation of ipRGCs causes cone photoreceptors to mislocalize at different apicobasal positions in the retina. This effect is partly mediated by light-evoked activity in ipRGCs, as dark rearing or silencing of ipRGCs leads a subset of cones to mislocalize. Furthermore, ablation of ipRGCs alters the cone transcriptome and decreases expression of the dopamine receptor D4, while injection of L-DOPA or D4 receptor agonist rescues the displaced cone phenotype observed in dark-reared animals. These results show that early light-mediated activity in ipRGCs influences neuronal lamination and identify ipRGC-elicited dopamine release as a mechanism influencing cone position.

Functional Role of SIL1 in Neurodevelopment and Learning

Background

Sil1 is the causative gene of Marinesco-Sjӧgren Syndrome (MSS). The mutated Sil1 generates shortened SIL1 protein which will form aggregation and be degraded rapidly. Mental retardation is a major symptom of MSS which suggests a role of SIL1 in the development of the central nervous system, but how SIL1 functions remains unclear.

Objectives

The aim of this study is to explore the role of SIL1 in regulating cerebral development and its underlying molecular mechanism.

Methods

The basic expression pattern of SIL1 in tissues and cultured cortical neurons is measured by immunostaining and Western blot. The expression of SIL1 is reduced in vitro and in vivo through RNA interference delivered by a lentivirus. The expression of NMDA receptor subunits and the function of the Reelin signaling pathway are then examined by surface biotinylation and Western blot subsequently. Finally, the spatial learning of young mice was assessed by the Barnes maze task.

Results

SIL1 deficiency caused a diminished expression of both Reelin receptors and therefore impaired the Reelin signaling pathway. It then inhibited the developmental expression of GluN2A and impaired the spatial learning of 5-week-old mice.

Conclusions

These results suggested that SIL1 is required for the development of the central nervous system which is associated with its role in Reelin signaling.

Maternal Deprivation Enhances Contextual Fear Memory via Epigenetically Programming Second-Hit Stress-Induced Reelin Expression in Adult Rats

BACKGROUND

Early-life stress increases the risk for posttraumatic stress disorder. However, the epigenetic mechanism of early-life stress-induced susceptibility to posttraumatic stress disorder in adulthood remains unclear.

METHODS

Rat pups were exposed to maternal deprivation during postnatal days 1 to 14 for 3 hours daily and treated with the DNA methyltransferase inhibitor zebularine, L-methionine, or vehicle 7 days before contextual fear conditioning, which was used as a second stress and to mimic the reexperiencing symptom of posttraumatic stress disorder in adulthood. Long-term potentiation, dendritic spine density, DNA methyltransferase mRNA, Reelin gene methylation, and Reelin protein expression in the hippocampal CA1 were measured.

RESULTS

Maternal deprivation enhanced contextual fear memory in adulthood. Meanwhile, maternal deprivation decreased DNA methyltransferase mRNA and Reelin gene methylation in the hippocampal CA1 on postnatal days 22 and 90. Reelin protein expression was increased in the hippocampal CA1 following contextual fear conditioning in adulthood. Furthermore, compared with rats that experienced maternal deprivation alone, rats also exposed to contextual fear conditioning showed an enhanced induction of hippocampal long-term potentiation and increased dendritic spine density in the hippocampal CA1 following contextual fear conditioning in adulthood. Zebularine pretreatment led to an enhancement of contextual fear memory, hypomethylation of the Reelin gene, and increased Reelin protein expression in adult rats, while L-methionine had the opposite effects.

CONCLUSIONS

Maternal deprivation can epigenetically program second-hit stress-induced Reelin expression and enhance the susceptibility to contextual fear memory in adulthood. These findings provide a new framework for understanding the cumulative stress hypothesis.

The ferroxidase ceruloplasmin influences Reelin processing, cofilin phosphorylation and neuronal organization in the developing brain

Ceruloplasmin (Cp) is an important extracellular regulator of iron metabolism. We showed previously that it stimulates Reelin proteolytic processing and cell aggregation in cultures of developing neurons. Reelin is a secreted protein required for the correct positioning of neurons in the brain. It is cleaved in vivo into N-terminally-derived 300K and 180K fragments through incompletely known mechanisms. One of Reelin signaling targets is the actin-binding protein cofilin, the phosphorylation of which is diminished in Reelin-deficient mice. This work looked for in vivo evidence of a relationship between Cp, Reelin and neuronal organization during brain development by analyzing wild-type and Cp-null mice. Cp as well as the full-length, 300K and 180K Reelin species appeared together in wild-type brains at embryonic day (E) 12.5 by immunoblotting. In wild-type compared to Cp-null brains, there was more 300K Reelin from E12.5 to E17.5, a period characterized by extensive, radially directed neuronal migration in the cerebral cortex. Immunofluorescence labeling of tissue sections at E16.5 revealed the localization of Cp with radial glia and meningeal cells adjacent to Reelin-producing Cajal-Retzius neurons, underlining the proximity of Cp and Reelin. Cofilin phosphorylation was seen starting at E10.5-E12.5 and lasted longer until postnatal day 7 in wild-type than Cp-null mice. Finally, using CUX1 as a marker revealed defective accumulation of neurons in layers II/III in neonatal and adult Cp-null mice. These results combined with our earlier work point to a potentially new role of Cp in Reelin processing and signaling and neuronal organization in the cerebral cortex in vivo.

Reelin Haploinsufficiency and Late-Adolescent Corticosterone Treatment Induce Long-Lasting and Female-Specific Molecular Changes in the Dorsal Hippocampus

Reelin depletion and stress seem to affect similar pathways including GABAergic and glutamatergic signaling and both are implicated in psychiatric disorders in late adolescence/early adulthood. The interaction between reelin depletion and stress, however, remains unclear. To investigate this, male and female heterozygous reelin mice (HRM) and wildtype (WT) controls were treated with the stress hormone, corticosterone (CORT), during late adolescence to simulate chronic stress. Glucocorticoid receptors (GR), N-methyl-d-aspartate receptor (NMDAr) subunits, glutamic acid decarboxylase (GAD₆₇) and parvalbumin (PV) were measured in the hippocampus and the prefrontal cortex (PFC) in adulthood. While no changes were seen in male mice, female HRM showed a significant reduction in GR expression in the dorsal hippocampus. In addition, CORT reduced GR levels as well as GluN2B and GluN2C subunits of NMDAr in the dorsal hippocampus in female mice only. CORT furthermore reduced GluN1 levels in the PFC of female mice. The combined effect of HRM and CORT treatment appeared to be additive in terms of GR expression in the dorsal hippocampus. Female-specific CORT-induced changes were associated with overall higher circulating CORT levels in female compared to male mice. This study shows differential effects of reelin depletion and CORT treatment on GR and NMDAr protein expression in male and female mice, suggesting that females are more susceptible to reelin haploinsufficiency as well as late-adolescent stress. These findings shed more light on female-specific vulnerability to stress and have implications for stress-associated mental illnesses with a female bias including anxiety and major depression.

Shaping Synapses by the Neural Extracellular Matrix

Accumulating data support the importance of interactions between pre- and postsynaptic neuronal elements with astroglial processes and extracellular matrix (ECM) for formation and plasticity of chemical synapses, and thus validate the concept of a tetrapartite synapse. Here we outline the major mechanisms driving: (i) synaptogenesis by secreted extracellular scaffolding molecules, like thrombospondins (TSPs), neuronal pentraxins (NPs) and cerebellins, which respectively promote presynaptic, postsynaptic differentiation or both; (ii) maturation of synapses via reelin and integrin ligands-mediated signaling; and (iii) regulation of synaptic plasticity by ECM-dependent control of induction and consolidation of new synaptic configurations. Particularly, we focused on potential importance of activity-dependent concerted activation of multiple extracellular proteases, such as ADAMTS4/5/15, MMP9 and neurotrypsin, for permissive and instructive events in synaptic remodeling through localized degradation of perisynaptic ECM and generation of proteolytic fragments as inducers of synaptic plasticity.

The functions of Reelin in membrane trafficking and cytoskeletal dynamics: implications for neuronal migration, polarization and differentiation

Reelin is a large extracellular matrix protein with relevant roles in mammalian central nervous system including neurogenesis, neuronal polarization and migration during development; and synaptic plasticity with its implications in learning and memory, in the adult. Dysfunctions in reelin signaling are associated with brain lamination defects such as lissencephaly, but also with neuropsychiatric diseases like autism, schizophrenia and depression as well with neurodegeneration. Reelin signaling involves a core pathway that activates upon reelin binding to its receptors, particularly ApoER2 (apolipoprotein E receptor 2)/LRP8 (low-density lipoprotein receptor-related protein 8) and very low-density lipoprotein receptor, followed by Src/Fyn-mediated phosphorylation of the adaptor protein Dab1 (Disabled-1). Phosphorylated Dab1 (pDab1) is a hub in the signaling cascade, from which several other downstream pathways diverge reflecting the different roles of reelin. Many of these pathways affect the dynamics of the actin and microtubular cytoskeleton, as well as membrane trafficking through the regulation of the activity of small GTPases, including the Rho and Rap families and molecules involved in cell polarity. The complexity of reelin functions is reflected by the fact that, even now, the precise mode of action of this signaling cascade in vivo at the cellular and molecular levels remains unclear. This review addresses and discusses in detail the participation of reelin in the processes underlying neurogenesis, neuronal migration in the cerebral cortex and the hippocampus; and the polarization, differentiation and maturation processes that neurons experiment in order to be functional in the adult brain. In vivo and in vitro evidence is presented in order to facilitate a better understanding of this fascinating system.

The fine tuning of retinocollicular topography depends on reelin signaling during early postnatal development of the rat visual system

During postnatal development, neural circuits are extremely dynamic and develop precise connection patterns that emerge as a result of the elimination of synaptic terminals, a process instructed by molecular cues and patterns of electrical activity. In the rodent visual system, this process begins during the first postnatal week and proceeds during the second and third postnatal weeks as spontaneous retinal activity and finally use-dependent fine tuning takes place. Reelin is a large extracellular matrix glycoprotein able to affect several steps of brain development, from neuronal migration to the maturation of dendritic spines and use-dependent synaptic development. In the present study, we investigated the role of reelin on the topographical refinement of primary sensory connections studying the development of retinal ganglion cell axon terminals in the rat superior colliculus. We found that reelin levels in the visual layers of the superior colliculus are the highest between the second and third postnatal weeks. Blocking reelin signaling with a neutralizing antibody (CR-50) from PND 7 to PND 14 induced a non-specific sprouting of ipsilateral retinocollicular axons outside their typical distribution of discrete patches of axon terminals. Also we found that reelin blockade resulted in reduced levels of phospho-GAP43, increased GluN1 and GluN2B-NMDA subunits and decreased levels of GAD65 content in the visual layers of the superior colliculus. The results suggest that reelin signaling is associated with the maturation of excitatory and inhibitory synaptic machinery influencing the development and fine tuning of topographically organized neural circuits during postnatal development.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

Reelin-Haploinsufficiency Disrupts the Developmental Trajectory of the E/I Balance in the Prefrontal Cortex

The reelin gene is a strong candidate in the etiology of several psychiatric disorders such as schizophrenia, major depression, bipolar disorders, and autism spectrum disorders. Most of these diseases are accompanied by cognitive and executive-function deficits associated with prefrontal dysfunctions. Mammalian prefrontal cortex (PFC) development is characterized by a protracted postnatal maturation constituting a period of enhanced vulnerability to psychiatric insults. The identification of the molecular components underlying this prolonged postnatal development is necessary to understand the synaptic properties of defective circuits participating in these psychiatric disorders. We have recently shown that reelin plays a key role in the maturation of glutamatergic functions in the postnatal PFC, but no data are available regarding the GABAergic circuits. Here, we undertook a cross-sectional analysis of GABAergic function in deep layer pyramidal neurons of the medial PFC of wild-type and haploinsufficient heterozygous reeler mice. Using electrophysiological approaches, we showed that decreased reelin levels impair the maturation of GABAergic synaptic transmission without affecting the inhibitory nature of GABA. This phenotype consequently impacted the developmental sequence of the synaptic excitation/inhibition (E/I) balance. These data indicate that reelin is necessary for the correct maturation and refinement of GABAergic synaptic circuits in the postnatal PFC and therefore provide a mechanism for altered E/I balance of prefrontal circuits associated with psychiatric disorders.

Reelin: Neurodevelopmental Architect and Homeostatic Regulator of Excitatory Synapses

Over half a century ago, D. S. Falconer first reported a mouse with a reeling gate. Four decades later, the Reln gene was isolated and identified as the cause of the reeler phenotype. Initial studies found that loss of Reelin, a large, secreted glycoprotein encoded by the Reln gene, results in abnormal neuronal layering throughout several regions of the brain. In the years since, the known functions of Reelin signaling in the brain have expanded to include multiple postdevelopmental neuromodulatory roles, revealing an ever increasing body of evidence to suggest that Reelin signaling is a critical player in the modulation of synaptic function. In writing this review, we intend to highlight the most fundamental aspects of Reelin signaling and integrate how these various neuromodulatory effects shape and protect synapses.

Interfering of the Reelin/ApoER2/PSD95 Signaling Axis Reactivates Dendritogenesis of Mature Hippocampal Neurons

Reelin, an extracellular glycoprotein secreted in embryonic and adult brain, participates in neuronal migration and neuronal plasticity. Extensive evidence shows that reelin via activation of the ApoER2 and VLDLR receptors promotes dendrite and spine formation during early development. Further evidence suggests that reelin signaling is needed to maintain a stable architecture in mature neurons, but, direct evidence is lacking. During activity-dependent maturation of the neuronal circuitry, the synaptic protein PSD95 is inserted into the postsynaptic membrane to induce structural refinement and stability of spines and dendrites. Given that ApoER2 interacts with PSD95, we tested if reelin signaling interference in adult neurons reactivates the dendritic architecture. Unlike findings in developing cultures, the presently obtained in vitro and in vivo data show, for the first time, that reelin signaling interference robustly increase dendritogenesis and reduce spine density in mature hippocampal neurons. In particular, the expression of a mutant ApoER2 form (ApoER2-tailless), which is unable to interact with PSD95 and hence cannot transduce reelin signaling, resulted in robust dendritogenesis in mature hippocampal neurons in vitro. These results indicate that reelin/ApoER2/PSD95 signaling is important for neuronal structure maintenance in mature neurons. Mechanistically, obtained immunofluorescent data indicate that reelin signaling impairment reduced synaptic PSD95 levels, consequently leading to synaptic re-insertion of NR2B-NMDARs. Our findings underscore the importance of reelin in maintaining adult network stability and reveal a new mode for reactivating dendritogenesis in neurological disorders where dendritic arbor complexity is limited, such as in depression, Alzheimer's disease, and stroke. J. Cell. Physiol. 232: 1187-1199, 2017. © 2016 Wiley Periodicals, Inc.

Multivariate synaptic and behavioral profiling reveals new developmental endophenotypes in the prefrontal cortex

The postnatal maturation of the prefrontal cortex (PFC) represents a period of increased vulnerability to risk factors and emergence of neuropsychiatric disorders. To disambiguate the pathophysiological mechanisms contributing to these disorders, we revisited the endophenotype approach from a developmental viewpoint. The extracellular matrix protein reelin which contributes to cellular and network plasticity, is a risk factor for several psychiatric diseases. We mapped the aggregate effect of the RELN risk allele on postnatal development of PFC functions by cross-sectional synaptic and behavioral analysis of reelin-haploinsufficient mice. Multivariate analysis of bootstrapped datasets revealed subgroups of phenotypic traits specific to each maturational epoch. The preeminence of synaptic AMPA/NMDA receptor content to pre-weaning and juvenile endophenotypes shifts to long-term potentiation and memory renewal during adolescence followed by NMDA-GluN2B synaptic content in adulthood. Strikingly, multivariate analysis shows that pharmacological rehabilitation of reelin haploinsufficient dysfunctions is mediated through induction of new endophenotypes rather than reversion to wild-type traits. By delineating previously unknown developmental endophenotypic sequences, we conceived a promising general strategy to disambiguate the molecular underpinnings of complex psychiatric disorders and for the rational design of pharmacotherapies in these disorders.

Identification of RELN variation p.Thr3192Ser in a Chinese family with schizophrenia

Schizophrenia (SCZ) is a serious psychiatric disease with strong heritability. Its complexity is reflected by extensive genetic heterogeneity and much of the genetic liability remains unaccounted for. We applied a combined strategy involving detection of copy number variants (CNVs), whole-genome mapping, and exome sequencing to identify the genetic basis of autosomal-dominant SCZ in a Chinese family. To rule out pathogenic CNVs, we first performed Illumina single nucleotide polymorphism (SNP) array analysis on samples from two patients and one psychiatrically healthy family member, but no pathogenic CNVs were detected. In order to further narrow down the susceptible region, we conducted genome-wide linkage analysis and mapped the disease locus to chromosome 7q21.13-22.3, with a maximum multipoint logarithm of odds score of 2.144. Whole-exome sequencing was then carried out with samples from three affected individuals and one unaffected individual in the family. A missense variation c.9575 C > G (p.Thr3192Ser) was identified in RELN, which is known as a risk gene for SCZ, located on chromosome 7q22, in the pedigree. This rare variant, as a highly penetrant risk variant, co-segregated with the phenotype. Our results provide genetic evidence that RELN may be one of pathogenic gene in SCZ.

Association between RELN Gene Polymorphisms and Attention Deficit Hyperactivity Disorder in Korean Children

OBJECTIVE

Attention deficit hyperactivity disorder (ADHD) is common disorder of the school-age population. ADHD is familial and genetic studies estimate heritability at 80-90%. The aim of the present study was to investigate the association between the genetic type and alleles for RELNgene (rs736707, rs2229864, rs362746, rs362726, rs362691, rs1062831, rs607755, and rs2072403) in Korean children with ADHD.

METHODS

The sample consisted of 180 ADHD children and 159 control children. We diagnosed ADHD according to DSM-IV. ADHD symptoms were evaluated with Conners' Parent Rating Scales and Dupaul Parent ADHD Rating Scales. Blood samples were taken from the 339 subjects, DNA was extracted from blood lymphocytes, and PCR was performed for RELN Polymorphism. Alleles and genotype frequencies were compared using the chi-square test. We compared the allele and genotype frequencies of RELN gene polymorphism in the ADHD and control groups.

RESULTS

This study showed that there was a significant correlation among the frequencies of the rs736707 (OR=1.40, 95% CI=1.03-1.90, p=0.031) of alleles of RELN, but the final conclusions are not definite.

CONCLUSION

Follow up studies with larger patient or pure subgroups are expected. These results suggested that RELN might be related to ADHD symptoms.

Reelin-Related Disturbances in Depression: Implications for Translational Studies

The finding that reelin expression is significantly decreased in mood and psychotic disorders, together with evidence that reelin can regulate key aspects of hippocampal plasticity in the adult brain, brought our research group and others to study the possible role of reelin in the pathogenesis of depression. This review describes recent progress on this topic using an animal model of depression that makes use of repeated corticosterone (CORT) injections. This methodology produces depression-like symptoms in both rats and mice that are reversed by antidepressant treatment. We have reported that CORT causes a decrease in the number of reelin-immunopositive cells in the dentate gyrus subgranular zone (SGZ), where adult hippocampal neurogenesis takes place; that down-regulation of the number of reelin-positive cells closely parallels the development of a depression-like phenotype during repeated CORT treatment; that reelin downregulation alters the co-expression of reelin with neuronal nitric oxide synthase (nNOS); that deficits in reelin might also create imbalances in glutamatergic and GABAergic circuits within the hippocampus and other limbic structures; and that co-treatment with antidepressant drugs prevents both reelin deficits and the development of a depression-like phenotype. We also observed alterations in the pattern of membrane protein clustering in peripheral lymphocytes in animals with low levels of reelin. Importantly, we found parallel changes in membrane protein clustering in depression patients, which differentiated two subpopulations of naïve depression patients that showed a different therapeutic response to antidepressant treatment. Here, we review these findings and develop the hypothesis that restoring reelin-related function could represent a novel approach for antidepressant therapies.

Multi-compartmental biomaterial scaffolds for patterning neural tissue organoids in models of neurodevelopment and tissue regeneration

Biomaterials are becoming an essential tool in the study and application of stem cell research. Various types of biomaterials enable three-dimensional culture of stem cells, and, more recently, also enable high-resolution patterning and organization of multicellular architectures. Biomaterials also hold potential to provide many additional advantages over cell transplants alone in regenerative medicine. This article describes novel designs for functionalized biomaterial constructs that guide tissue development to targeted regional identities and structures. Such designs comprise compartmentalized regions in the biomaterial structure that are functionalized with molecular factors that form concentration gradients through the construct and guide stem cell development, axis patterning, and tissue architecture, including rostral/caudal, ventral/dorsal, or medial/lateral identities of the central nervous system. The ability to recapitulate innate developmental processes in a three-dimensional environment and under specific controlled conditions has vital application to advanced models of neurodevelopment and for repair of specific sites of damaged or diseased neural tissue.

Losing the sugar coating: potential impact of perineuronal net abnormalities on interneurons in schizophrenia

Perineuronal nets (PNNs) were shown to be markedly altered in subjects with schizophrenia. In particular, decreases of PNNs have been detected in the amygdala, entorhinal cortex and prefrontal cortex. The formation of these specialized extracellular matrix (ECM) aggregates during postnatal development, their functions, and association with distinct populations of GABAergic interneurons, bear great relevance to the pathophysiology of schizophrenia. PNNs gradually mature in an experience-dependent manner during late stages of postnatal development, overlapping with the prodromal period/age of onset of schizophrenia. Throughout adulthood, PNNs regulate neuronal properties, including synaptic remodeling, cell membrane compartmentalization and subsequent regulation of glutamate receptors and calcium channels, and susceptibility to oxidative stress. With the present paper, we discuss evidence for PNN abnormalities in schizophrenia, the potential functional impact of such abnormalities on inhibitory circuits and, in turn, cognitive and emotion processing. We integrate these considerations with results from recent genetic studies showing genetic susceptibility for schizophrenia associated with genes encoding for PNN components, matrix-regulating molecules and immune system factors. Notably, the composition of PNNs is regulated dynamically in response to factors such as fear, reward, stress, and immune response. This regulation occurs through families of matrix metalloproteinases that cleave ECM components, altering their functions and affecting plasticity. Several metalloproteinases have been proposed as vulnerability factors for schizophrenia. We speculate that the physiological process of PNN remodeling may be disrupted in schizophrenia as a result of interactions between matrix remodeling processes and immune system dysregulation. In turn, these mechanisms may contribute to the dysfunction of GABAergic neurons.

Reelin protects against amyloid β toxicity in vivo

Alzheimer's disease (AD) is a currently incurable neurodegenerative disorder and is the most common form of dementia in people over the age of 65 years. The predominant genetic risk factor for AD is the ε4 allele encoding apolipoprotein E (ApoE4). The secreted glycoprotein Reelin enhances synaptic plasticity by binding to the multifunctional ApoE receptors apolipoprotein E receptor 2 (Apoer2) and very low density lipoprotein receptor (Vldlr). We have previously shown that the presence of ApoE4 renders neurons unresponsive to Reelin by impairing the recycling of the receptors, thereby decreasing its protective effects against amyloid β (Aβ) oligomer-induced synaptic toxicity in vitro. We showed that when Reelin was knocked out in adult mice, these mice behaved normally without overt learning or memory deficits. However, they were strikingly sensitive to amyloid-induced synaptic suppression and had profound memory and learning disabilities with very low amounts of amyloid deposition. Our findings highlight the physiological importance of Reelin in protecting the brain against Aβ-induced synaptic dysfunction and memory impairment.

Reelin supplementation recovers synaptic plasticity and cognitive deficits in a mouse model for Angelman syndrome

The Reelin signaling pathway is implicated in processes controlling synaptic plasticity and hippocampus-dependent learning and memory. A single direct in vivo application of Reelin enhances long-term potentiation, increases dendritic spine density and improves associative and spatial learning and memory. Angelman syndrome (AS) is a neurological disorder that presents with an overall defect in synaptic function, including decreased long-term potentiation, reduced dendritic spine density, and deficits in learning and memory, making it an attractive model in which to examine the ability of Reelin to recover synaptic function and cognitive deficits. In this study, we investigated the effects of Reelin administration on synaptic plasticity and cognitive function in a mouse model of AS and demonstrated that bilateral, intraventricular injections of Reelin recover synaptic function and corresponding hippocampus-dependent associative and spatial learning and memory. Additionally, we describe alteration of the Reelin profile in tissue from both the AS mouse and post-mortem human brain.

The effect of chicken extract on ERK/CREB signaling is ApoE isoform-dependent

It is unclear how the nutritional supplement chicken extract (CE) enhances cognition. Human apolipoprotein E (ApoE) can regulate cognition and this isoform-dependent effect is associated with the N-methyl-d-aspartate receptor (NMDAR). To understand if CE utilizes this pathway, we compared the NMDAR signaling in neuronal cells expressing ApoE3 and ApoE4. We observed that CE increased S896 phosphorylation on NR1 in ApoE3 cells and this was linked to higher protein kinase C (PKC) activation. However, ApoE4 cells treated with CE have lowered S897 phosphorylation on NR1 and this was associated with reduced protein kinase A (PKA) phosphorylation. In ApoE3 cells, CE increased calmodulin kinase II (CaMKII) activation and AMPA GluR1 phosphorylation on S831. In contrast, CE reduced CaMKII phosphorylation and led to higher de-phosphorylation of S831 and S845 on GluR1 in ApoE4 cells. While CE enhanced ERK/CREB phosphorylation in ApoE3 cells, this pathway was down-regulated in both ApoE4 and mock cells after CE treatment. These results show that CE triggers ApoE isoform-specific changes on ERK/CREB signaling.

Reelin has a preventive effect on phencyclidine-induced cognitive and sensory-motor gating deficits

Reelin has recently attracted attention because of its connection to several neuropsychiatric diseases. We previously reported the finding that prior transplantation of GABAergic neuron precursor cells into the medial prefrontal cortex (mPFC) of mice significantly prevented the induction of cognitive and sensory-motor gating deficits induced by phencyclidine (PCP). The majority of the precursor cells transplanted into the mPFC of the recipient mice differentiated into members of a somatostatin/Reelin-expressing class of GABAergic interneurons. These findings raised the possibility that Reelin secreted by the transplanted cells plays an important role in preventing the deficits induced by PCP. In this study, we investigated whether Reelin itself has a preventive effect on PCP-induced behavioral phenotypes by injecting conditioned medium containing Reelin into the lateral ventricle of the brains of 6- to 7-week-old male mice before administrating PCP. Behavioral analyses showed that the prior Reelin injection had a preventive effect against induction of the cognitive and sensory-motor gating deficits associated with PCP. Moreover, one of the types of Reelin receptor was found to be expressed by neurons in the mPFC. The results of this study point to the Reelin signaling pathway as a candidate target for the pharmacologic treatment of neuropsychiatric diseases.

MMPs: a novel drug target for schizophrenia

INTRODUCTION

Schizophrenia, a multifactorial disorder, is associated with dopaminergic hyperactivity, dysregulated glutamatergic neurotransmission, neuroinflammation and extracellular matrix (ECM) disturbances. MMPs, a group of structurally related proteolytic enzymes, are responsible for remodeling of ECM that maintains synaptic functions and blood-brain barrier (BBB) patency. Overstimulation of MMPs by neuroinflammation triggers ECM abnormalities that directly or indirectly alter neuronal functions like synaptic plasticity and damage to BBB. MMP-mediated ECM abnormality plays a central role in the pathogenesis of schizophrenia.

AREAS COVERED

The current review discusses the mechanistic involvement of MMPs in the pathogenesis of schizophrenia and briefly gives an overview on the recent studies on various MMP modulators.

EXPERT OPINION

Overexpression of MMPs and imbalance between MMP versus tissue inhibitors of metalloproteinase are associated with various ECM disturbances in the schizophrenic brain. Therefore, MMPs can be projected as potential therapeutic target for treatment and/or prevention of positive, negative and cognitive symptoms of schizophrenia. From past decade, scientific community is focusing on broad spectrum MMP modulators as potential therapeutic moieties for prevention of plethora of neurological, cardiovascular and pulmonary diseases. In future, specific MMP modulators should be tailored to regulate ECM integrity and explored for their pharmacotherapeutic potential in schizophrenia.

Corticosterone treatment during adolescence induces down-regulation of reelin and NMDA receptor subunit GLUN2C expression only in male mice: implications for schizophrenia

Stress exposure during adolescence/early adulthood has been shown to increase the risk for psychiatric disorders such as schizophrenia. Reelin plays an essential role in brain development and its levels are decreased in schizophrenia. However, the relationship between stress exposure and reelin expression remains unclear. We therefore treated adolescent reelin heteroyzogous mice (HRM) and wild-type (WT) littermates with the stress hormone, corticosterone (CORT) in their drinking water (25 mg/l) for 3 wk. In adulthood, we measured levels of full-length (FL) reelin and the N-R6 and N-R2 cleavage fragments in the frontal cortex (FC) and dorsal (DH) and ventral (VH) hippocampus. As expected, levels of all reelin forms were approximately 50% lower in HRMs compared to WT. In male mice, CORT treatment significantly decreased FL and N-R2 expression in the FC and N-R2 and N-R6 levels in the DH. This reelin down-regulation was accompanied by significant reductions in downstream N-methyl-D-aspartate (NMDA) GluN2C subunit levels. There were no effects of CORT treatment in the VH of either of the sexes and only subtle changes in female DH. CORT-induced reelin and GluN2C down-regulation in males was not associated with changes in two GABAergic neuron markers, GAD67 and parvalbumin, or glucocorticoids receptors (GR). These results show that CORT treatment causes long-lasting and selective reductions of reelin form levels in male FC and DH accompanied by changes in NMDAR subunit composition. This sex-specific reelin down-regulation in regions implicated in schizophrenia could be involved in the effects of stress in this disease.

Alterations in the hippocampal and striatal catecholaminergic fiber densities of heterozygous reeler mice

The heterozygous reeler mouse (HRM), haploinsufficient for reelin, shares several neurochemical and behavioral similarities with patients suffering from schizophrenia. It has been shown that defective reelin signaling influences the mesolimbic dopaminergic pathways in a specific manner. However, there is only little information about the impact of reelin haploinsufficiency on the monoaminergic innervation of different brain areas, known to be involved in the pathophysiology of schizophrenia. In the present study using immunocytochemical procedures, we investigated HRM and wild-type mice (WT) for differences in the densities of tyrosine hydroxylase (TH)-immunoreactive (IR) and serotonin (5-HT)-IR fibers in prefrontal cortex, ventral and dorsal hippocampal formation, amygdala and ventral and dorsal striatum. We found that HRM, compared to WT, shows a significant increase in TH-IR fiber densities in dorsal hippocampal CA1, CA3 and ventral CA1. In contrast, HRM exhibits a significant decrease of TH-IR in the shell of the nucleus accumbens (AcbShell), but no differences in the other brain areas investigated. Overall, no genotype differences were found in the 5-HT-IR fiber densities. In conclusion, these results support the view that reelin haploinsufficiency differentially influences the catecholaminergic (esp. dopaminergic) systems in brain areas associated with schizophrenia. The reelin haploinsufficient mouse may provide a useful model for studying the role of reelin in hippocampal dysfunction and its effect on the dopaminergic system as related to schizophrenia.

Reelin induces Erk1/2 signaling in cortical neurons through a non-canonical pathway

Reelin is an extracellular protein that controls many aspects of pre- and postnatal brain development and function. The molecular mechanisms that mediate postnatal activities of Reelin are not well understood. Here, we first set out to express and purify the full length Reelin protein and a biologically active central fragment. Second, we investigated in detail the signal transduction mechanisms elicited by these purified Reelin proteins in cortical neurons. Unexpectedly, we discovered that the full-length Reelin moiety, but not the central fragment, is capable of activating Erk1/2 signaling, leading to increased p90RSK phosphorylation and the induction of immediate-early gene expression. Remarkably, Erk1/2 activation is not mediated by the canonical signal transduction pathway, involving ApoER2/VLDLR and Dab1, that mediates other functions of Reelin in early brain development. The activation of Erk1/2 signaling likely contributes to the modulation of neuronal maturation and synaptic plasticity by Reelin in the postnatal and adult brain.

Sorting nexin 17 regulates ApoER2 recycling and reelin signaling

ApoER2 is a member of the low density-lipoprotein receptor (LDL-R) family. As a receptor for reelin, ApoER2 participates in neuronal migration during development as well as synaptic plasticity and survival in the adult brain. A previous yeast two-hybrid screen showed that ApoER2 is a binding partner of sorting nexin 17 (SNX17) - a cytosolic adaptor protein that regulates the trafficking of several membrane proteins in the endosomal pathway, including LRP1, P-selectin and integrins. However, no further studies have been performed to investigate the role of SNX17 in ApoER2 trafficking and function. In this study, we present evidence based on GST pull-down and inmunoprecipitation assays that the cytoplasmic NPxY endocytosis motif of ApoER2 interacts with the FERM domain of SNX17. SNX17 stimulates ApoER2 recycling in different cell lines including neurons without affecting its endocytic rate and also facilitates the transport of ApoER2 from the early endosomes to the recycling endosomes. The reduction of SNX17 was associated with accumulation of an ApoER2 carboxy-terminal fragment (CTF). In addition, in SNX17 knockdown cells, constitutive ApoER2 degradation was not modified, whereas reelin-induced ApoER2 degradation was increased, implying that SNX17 is a regulator of the receptor's half-life. Finally, in SNX17 silenced hippocampal and cortical neurons, we underscored a positive role of this endosomal protein in the development of the dendritic tree and reelin signaling. Overall, these results establish the role of SNX17 in ApoER2 trafficking and function and aid in identifying new links between endocytic trafficking and receptor signaling.

Reelin, an extracellular matrix protein linked to early onset psychiatric diseases, drives postnatal development of the prefrontal cortex via GluN2B-NMDARs and the mTOR pathway

Defective brain extracellular matrix (ECM) is a factor of vulnerability in various psychiatric diseases such as schizophrenia, depression and autism. The glycoprotein reelin is an essential building block of the brain ECM that modulates neuronal development and participates to the functions of adult central synapses. The reelin gene (RELN) is a strong candidate in psychiatric diseases of early onset, but its synaptic and behavioral functions in juvenile brain circuits remain unresolved. Here, we found that in juvenile reelin-haploinsufficient heterozygous reeler mice (HRM), abnormal fear memory erasure is concomitant to reduced dendritic spine density and anomalous long-term potentiation in the prefrontal cortex. In juvenile HRM, a single in vivo injection with ketamine or Ro25-6981 to inhibit GluN2B-N-methyl-D-aspartate receptors (NMDARs) restored normal spine density, synaptic plasticity and converted fear memory to an erasure-resilient state typical of adult rodents. The functional and behavioral rescue by ketamine was prevented by rapamycin, an inhibitor of the mammalian target of rapamycin pathway. Finally, we show that fear memory erasure persists until adolescence in HRM and that a single exposure to ketamine during the juvenile period reinstates normal fear memory in adolescent mice. Our results show that reelin is essential for successful structural, functional and behavioral development of juvenile prefrontal circuits and that this developmental period provides a critical window for therapeutic rehabilitation with GluN2B-NMDAR antagonists.

Reelin in the Years: Controlling Neuronal Migration and Maturation in the Mammalian Brain

The extracellular protein Reelin was initially identified as an essential factor in the control of neuronal migration and layer formation in the developing mammalian brain. In the years following its discovery, however, it became clear that Reelin is a multifunctional protein that controls not only the positioning of neurons in the developing brain, but also their growth, maturation, and synaptic activity in the adult brain. In this review, we will highlight the major discoveries of the biological activities of Reelin and the underlying molecular mechanisms that affect the development and function of the mammalian brain, from embryonic ages to adulthood.

Apolipoprotein E and apolipoprotein E receptors: normal biology and roles in Alzheimer disease

Apolipoprotein E (APOE) genotype is the major genetic risk factor for Alzheimer disease (AD); the ε4 allele increases risk and the ε2 allele is protective. In the central nervous system (CNS), apoE is produced by glial cells, is present in high-density-like lipoproteins, interacts with several receptors that are members of the low-density lipoprotein receptor (LDLR) family, and is a protein that binds to the amyloid-β (Aβ) peptide. There are a variety of mechanisms by which apoE isoform may influence risk for AD. There is substantial evidence that differential effects of apoE isoform on AD risk are influenced by the ability of apoE to affect Aβ aggregation and clearance in the brain. Other mechanisms are also likely to play a role in the ability of apoE to influence CNS function as well as AD, including effects on synaptic plasticity, cell signaling, lipid transport and metabolism, and neuroinflammation. ApoE receptors, including LDLRs, Apoer2, very low-density lipoprotein receptors (VLDLRs), and lipoprotein receptor-related protein 1 (LRP1) appear to influence both the CNS effects of apoE as well as Aβ metabolism and toxicity. Therapeutic strategies based on apoE and apoE receptors may include influencing apoE/Aβ interactions, apoE structure, apoE lipidation, LDLR receptor family member function, and signaling. Understanding the normal and disease-related biology connecting apoE, apoE receptors, and AD is likely to provide novel insights into AD pathogenesis and treatment.

The brain's extracellular matrix and its role in synaptic plasticity

The extracellular matrix (ECM) of the brain has important roles in regulating synaptic function and plasticity. A juvenile ECM supports the wiring of neuronal networks, synaptogenesis, and synaptic maturation. The closure of critical periods for experience-dependent shaping of neuronal circuits coincides with the implementation of a mature form of ECM that is characterized by highly elaborate hyaluronan-based structures, the perineuronal nets (PNN), and PNN-like perisynaptic ECM specializations. In this chapter, we will focus on some recently reported aspects of ECM functions in brain plasticity. These include (a) the discovery that the ECM can act as a passive diffusion barrier for cell surface molecules including neurotransmitter receptors and in this way compartmentalize cell surfaces, (b) the specific functions of ECM components in actively regulating synaptic plasticity and homeostasis, and (c) the shaping processes of the ECM by extracellular proteases and in turn the activation particular signaling pathways.

Altered N-methyl-D-aspartate receptor function in reelin heterozygous mice: male-female differences and comparison with dopaminergic activity

The aim of this study was to investigate the in vivo relationship between reelin and NMDA receptor function in schizophrenia. We assessed the effect of reelin deficiency in behavioral models of aspects of this illness, NMDA receptor subunit levels, and NMDA receptor, dopamine D₂ receptor, and dopamine transporter density. Male, but not female, reelin heterozygous mice showed significantly enhanced MK-801-induced locomotor hyperactivity compared to wildtype controls (7.4-fold vs. 5.2-fold effect of MK-801 over saline, respectively) but there were no genotype differences in the response to amphetamine. Both male and female reelin heterozygous mice showed enhanced effects of MK-801 on startle, but not prepulse inhibition (PPI) of startle. There were no group differences in the effect of apomorphine on startle or PPI. The levels of NMDA receptor subunits were not altered in the striatum. In the frontal cortex, male and female reelin heterozygous mice showed significant up-regulation of NR1 subunits, but down-regulation of NR2C subunits, which was associated with significantly elevated NR1/NR2A and NR1/NR2C ratios. However, there were no differences in [³H]MK-801 binding density in the nucleus accumbens or caudate nucleus, nor in the density of [³H]YM-09151 or [³H]GBR12935 in these brain regions. The enhanced effects of MK-801 in reelin heterozygous mice in this study could be reflective of the role of reelin deficiency in schizophrenia. This genotype effect was male-specific for locomotor hyperactivity, a model of psychosis, but was seen in male and female mice for startle, which could be an indication of changes in anxiety. Changes in NMDA receptor subunit levels and ratios were also seen in both male and female mice. These results suggest that the role of reelin deficiency in schizophrenia may be particularly mediated by altered NMDA receptor responses, with some of these effects being strictly sex-specific.

PCSK9 regulates neuronal apoptosis by adjusting ApoER2 levels and signaling

The secreted protease proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to low-density lipid (LDL) receptor family members LDLR, very low density lipoprotein receptor (VLDLR) and apolipoprotein receptor 2 (ApoER2), and promotes their degradation in intracellular acidic compartments. In the liver, LDLR is a major controller of blood LDL levels, whereas VLDLR and ApoER2 in the brain mediate Reelin signaling, a critical pathway for proper development of the nervous system. Expression level of PCSK9 in the brain is highest in the cerebellum during perinatal development, but is also increased in the adult brain after ischemia. The mechanism of PCSK9 function and its involvement in neuronal apoptosis is poorly understood. We show here that RNAi-mediated knockdown of PCSK9 significantly reduced the death of potassium-deprived cerebellar granule neurons (CGN), as shown by reduced levels of nuclear phosphorylated c-Jun and activated caspase-3, as well as condensed apoptotic nuclei. ApoER2 protein levels were increased in PCSK9 RNAi cells. Knockdown of ApoER2 but not of VLDLR was sufficient to reverse the protection provided by PCSK9 RNAi, suggesting that proapoptotic signaling of PCSK9 is mediated by altered ApoER2 function. Pharmacological inhibition of signaling pathways associated with lipoprotein receptors suggested that PCSK9 regulates neuronal apoptosis independently of NMDA receptor function but in concert with ERK and JNK signaling pathways. PCSK9 RNAi also reduced staurosporine-induced CGN apoptosis and axonal degeneration in the nerve growth factor-deprived dorsal root ganglion neurons. We conclude that PCSK9 potentiates neuronal apoptosis via modulation of ApoER2 levels and related anti-apoptotic signaling pathways.